1. Field of the Invention
The present invention relates to an exhaust gas-purifying catalyst, and in particular to a motor vehicle exhaust gas-purifying catalyst having a carrier comprising a zirconium-based composite oxide.
2. Description of the Related Art
In recent years, the regulation of exhaust gas of motor vehicles is being made stricter, increasing the necessity for further decreasing the amounts of hydrocarbons (HC), carbon monoxide (CO) and nitrogen oxides (NOx) in the exhaust gases.
Conventionally, a carried catalyst in which a noble metal (catalyst active component) such as rhodium (Rh) is carried on a porous carrier has been widely employed as an exhaust gas-purifying catalyst for motor vehicles. Such a carried catalyst is called a three-way catalyst because it can oxidize CO and HC and reduce NOx in the exhaust gas simultaneously.
Conventionally, γ-alumina is used as the carrier. However, γ-alumina undergoes phase-transition to α-alumina during use at high temperatures for a long period of time, and consequently the specific surface area of γ-alumina decreases. Further, there is also a problem that rhodium is solid-solutioned into the alumina during use at high temperatures, and the catalyst performances are lowered (see Jpn. Pat. Appln. KOKAI Publication No. 2001-347167).
Under the circumstances, carriers superior in heat resistance are required, instead of alumina. For example, Jpn. Pat. Appln. KOKAI Publication No. 2000-15101 discloses carrying rhodium on a zirconia support stabilized with an alkaline earth metal. Further, Jpn. Pat. Appln. KOKAI Publication No. 2002-518171 (corresponding to International Publication WO 99/67020) discloses a catalyst in which rhodium is carried on a zirconia support stabilized with a rare earth element.
In these prior arts, as the stabilized zirconia materials carrying rhodium, those having a large specific surface area are employed and rhodium is highly dispersed and carried on the surface of the zirconia.
However, the rhodium carried on the stabilized zirconia carrier having such a large specific surface area grows in its grains during use at high temperatures, accompanying the sintering of rhodium and embedding of the rhodium into the carrier particles, and the catalytic activity is lowered.
Accordingly, an object of the present invention is to provide an exhaust gas-purifying catalyst which is suppressed in lowering of the catalytic activity during use at high temperatures, even though it uses a stabilized zirconia-based composite oxide such as stabilized zirconia as the carrier material.